High temperature degradation of ohmic contacts on p-GaN

This paper analyzes the high temperature instabilities of ohmic contacts on p-type gallium nitride to be used for GaN-based LEDs. During the last years a few authors have demonstrated GaN-based LEDs failure during high temperature stress. In particular, it has been demonstrated that the effect of high temperatures is to induce the decrease of the LEDs optical power, well related to the increase of the operating voltage. The effect has been attributed to the degradation of the p-side and p-contacts of the diodes, enhanced by the interaction between Mg-acceptor and the hydrogen contained in the PECVD SiN passivation layer usually deposited on the LEDs for chips encapsulation and leakage reduction. However, previous studies did not describe how stress affects the properties of the metal/semiconductor system, due to the lack of specific test structures. This work completes the current understanding of the high temperature degradation of the electrical properties of GaN LEDs, describing how the ohmic contacts and the semiconductor material degrade during thermal stress. The contributions of the ohmic contacts and semiconductor material degradation are separated by means of the Transfer Length Method (TLM).